1. Field of the Invention
The invention relates to a flexible board capable of preventing generation of unnecessary radiation therefrom, a method of fabricating such a flexible board, and a cellular phone including such a flexible board.
2. Description of the Related Art
A cellular phone has been required to be light in weight, small in size, and thin in thickness. In order to meet such requirement, various parts to be incorporated into a body of a cellular phone are also required to be light, small and thin. In addition, since unnecessary radiation might exert harmful influence on sensitivity of a cellular phone on receiving radio signals, a cellular phone is required not to be influenced by unnecessary radiation.
The above-mentioned unnecessary radiation is radiated a lot from a bus line and a clock line among memories and controllers, for instance. If what unnecessary radiation radiates such as the above-mentioned bus line and clock line is formed on a flexible board, it would be quite difficult to prevent unnecessary radiation from radiating from what unnecessary radiation radiates. Hence, only limited kinds of lines are allowed to be formed on a flexible board.
In order to avoid this problem, a flexible board has been conventionally designed to have a multi-layered printed structure, and what unnecessary radiation radiates a lot such as a bus line and a clock line has been formed on an internal layer in the multi-layered printed structure to thereby minimize generation of unnecessary radiation therefrom. A line which is likely to be influenced by unnecessary radiation has been conventionally formed also on an internal layer in the multi-layered printed structure.
However, if a bus line and a clock line are to be arranged on an internal layer in the multi-layered printed structure, it would be unavoidable for the multi-layered printed structure to become large in order to ensure a space for arranging those lines. If the multi-layered printed structure is large in size, it would be impossible to incorporate the flexible board into a cellular phone required to be as small as possible.
Japanese Unexamined Patent Publication No. 5-3395 has suggested a flexible printed board including a circuit sandwiched between flexible insulating layers. A shield layer composed of metal and having a thickness equal to or smaller than 5 μm is formed on a surface of one of the flexible insulating layers. A second insulating layer for fixing the shield layer onto the flexible insulating layers is adhered onto both the shield layer and the flexible insulating layers through adhesive layers.
Japanese Unexamined Patent Publication No. 7-111371 has suggested a flexible printed board including a first insulating layer formed at a predetermined location with a first contact hole, a wiring layer, a second insulating layer formed at a predetermined location with a second contact hole, a first electromagnetic shield layer comprised of a third insulating layer and a first electrically conductive layer such that the first electrically conductive layer makes contact with the first insulating layer, the first electromagnetic shield layer being formed with a first through-hole above the first contact hole, the wiring layer and the first electromagnetic shield layer being electrically connected to each other through electrical conductor filled in the first contact hole and the first through hole, an adhesive layer formed around the electrical conductor between the first insulating layer and the first electrically conductive layer, a second electromagnetic shield layer comprised of a fourth insulating layer and a second electrically conductive layer such that the second electrically conductive layer makes contact with the second insulating layer, the second electromagnetic shield layer being formed with a second through-hole above the second contact hole, the wiring layer and the second electromagnetic shield layer being electrically connected to each other through electrical conductor filled in the second contact hole and the second through hole, and an adhesive layer formed around the electrical conductor between the second insulating layer and the second electrically conductive layer.
Japanese Unexamined Patent Publication No. 8-122806 has suggested a liquid crystal display device having a multi-layered flexible substrate. The suggested liquid crystal display device is comprised of a chip-on-glass type liquid crystal display unit including two transparent insulating substrates overlapping each other, a drive integrated circuit being mounted on one of the transparent insulating substrates, and a multi-layered flexible substrate having three or more electrically conductive layers, electrically connected to an input terminal pattern of the drive integrated circuit, and arranged outside at least on side of the liquid crystal display unit.
Japanese Unexamined Patent Publication No. 9-138388 has suggested a liquid crystal display device including a liquid crystal display unit and a first circuit substrate electrically connected to the liquid crystal display unit. The first circuit substrate partially overlaps the liquid crystal display unit as viewed perpendicularly to the first circuit substrate.
Japanese Unexamined Patent Publication No. 9-197425 has suggested a liquid crystal display device including a liquid crystal display panel, a circuit board for driving liquid crystal, divided into at least two pieces, an electrically conductive layer supported by a substrate layer, and a joiner for electrically connecting the divided circuit boards to each other. The joiner is comprised of a signal layer contributing to electrical connection of the divided circuit boards, and a ground layer.
Japanese Unexamined Patent Publication No. 10-74550 has suggested a liquid crystal display device comprised of a first circuit board, a second circuit board, and a connector for electrically connecting the first and second circuit boards to each other, an electrical conductor covering an outer surface of the connector. At least one of ground lines or a power supply line of the first and second circuit boards is electrically connected to the electrical conductor.
Japanese Unexamined Patent Publication No. 10-133218 has suggested a tape carrier package on which IC chip is mounted. The tape carrier package has a base film at a terminal thereof for preventing scattering, and a wiring is formed on the base film.
Japanese Unexamined Patent Publication No. 7-170089 has suggested a flexible printed wiring board comprising a substrate composed of an electrically insulating film and formed at a surface thereof with an electrically conductive path, an electrically insulating layer covering the electrically conductive path therewith, an external layer composed of an electrical conductor having a greater size than that of said electrically conductive path and formed on the electrically insulating layer and the substrate, and couplers extending throughout the substrate to thereby electrically couple the external layers to each other at a location out of the electrically conductive path, the couplers being arranged at a pitch smaller than a predetermined pitch in a direction in which a signal is transmitted.